1. Field of the Invention
This invention relates generally to the control of cardiac pacemakers, and more particularly to a rate responsive controller adapted to automatically adjust the pacing rate of a cardiac pacemaker responsive to variations in a patient's intracardiac blood temperature.
2. Description of Related Art
Fixed rate, synchronous and demand pacers designed to pace the heart at a predetermined rate associated with a low or moderate level of physical activity are well known. Such pacers are not designed to adjust the pacing rate to provide the increased level of cardiac response needed to support increased physical activity such as work or exercise. As a result, patients using such pacemakers are constrained in the amount of physical activity they may undertake.
A number of pacemaker and pacer control systems have been proposed in the past to address this shortcoming. One group of such systems of particular interest have proposed using various physiological parameters known to vary with physical activity to provide exercise-responsive adjustments to the pacing rate. For example, systems responsive to the oxygen content of the blood (U.S. Pat. Nos. 4,399,820 and 4,202,339 to Wirtzfield), to blood pressure (U.S. Pat. No. 3,828,371 to Purdy; U.S. Pat. No. 3,638,656 to Grandjean), to respiratory volume (U.S. Pat. No. 3,593,718 to Krasner), and to blood pH (U.S. Pat. No. 4,009,721 to Alcidi) have all been proposed to provide exercise-responsive control of pacing rate.
The proposed systems have a number of problems and drawbacks that have limited their acceptance and use. For instance, it has been found that some of the proposed physiological parameters can vary in response to stimuli such as certain medications in addition to increased physical activity, thus occasionally leading to unintended rate response variations. Another problem has been the difficulty in designing a sensor to accurately measure the proposed parameters. Still another problem has been that sensors designed to measure the parameters of interest have been found unsuitable for long term implantation.
Intracardiac blood temperature has also been proposed as a parameter indicative of physical activity to control pacing rate. It has been found that blood temperature provides a more accurate and reliable indication of physical activity than other proposed parameters and that it may be sensed accurately and easily with readily available temperature sensors.
U.S. Pat. Nos. 4,436,092 and 4,543,954 to Cook for example disclose rate responsive pacers in which intracardiac blood temperature is detected by a thermistor inserted intravenously in the right ventricle. The Cook patents relate the detected temperature to a pacing rate by a mathematical formula derived from the experimentally observed relation between heart rate and intracardiac blood temperature in test dogs. The mathematical formula includes a resting rate term, a term related to the difference between current temperature and a fixed reference temperature, and a term related to the rate of change of temperature with respect to time. The Cook '954 patent constrains the calculated pacing rate to one of three discrete values, a high exercise value, a low at-rest value, and an intermediate value.
German Patent No. GM 7606824 to Csapo discloses another temperature responsive pacemaker which utilizes a thermistor located in the heart to control the oscillation rate of a blocking oscillator. The blocking oscillator accordingly generates pacing pulses at a rate related to the intracardiac blood temperature. In another embodiment, the thermistor controls the state of a multivibrator to provide discrete pacing rate levels similar to the Cook '954 patent.
The known temperature responsive pacers, although comprising an improvement over other proposed rate responsive systems, are also subject to a number of drawbacks and deficiencies. For example, studies of the relationship between intracardiac blood temperature and heart rate response in patients with normally functioning hearts indicate (1) that the intracardiac blood temperature and the heart rate vary naturally in a small range over a circadian cycle as well as with fever and the like, and (2) that the beat rate of a normally functioning heart is gradually adaptive to varying levels of physical activity. The rate response of the Cook pacers, however, is based on a single temperature versus time slope term. If the magnitude of the slope term is set to provide adequate exercise response, then natural temperature variations due to fever, the circadian cycle, and the like produce inappropriately large pacing rate adjustments. Therefore, the magnitude of the slope term is designed to be relatively small to provide appropriate rate response for non-activity related temperature variations. Exercise response is provided in the form of a large abrupt addition to the pacing rate.
Also, the Cook algorithm is a function of a single, fixed reference temperature. As a result, the algorithm can produce inappropriate pacing rate calculations if the reference temperature input to the system and used by the algorithm is itself inaccurate and also as the patient's resting blood temperature changes significantly throughout the day.
The prior art rate responsive systems have also lacked various features that provide improved accuracy and safety. For example, one desirable feature not found in the prior art systems is means to control the slew rate or rate of change of the pacing rate. Such control means eliminate abrupt rate variations, which can be harmful to the patient in certain situations, and ensure smooth rate response.
Another desirable feature not found in the prior art systems is means to prevent the rate saturation effect that can occur when the rate response algorithm is capable of providing greater rate response than the desired maximum pacing rate. This can result in the undesirable condition of the pacemaker continuing to pace the heart at the maximum rate even for a period of time after the patient has ceased the physical activity that gave rise to the increased rate response in the first place.
Yet another desirable feature not found in the prior art systems, particularly those such as the Cook '092 system that calculate the rate response as a function of a fixed reference temperature, is means to automatically adjust the reference temperature internally to compensate for any error in the initially supplied value and for naturally occurring changes in the resting temperature. Such adjustment means improves the accuracy of the rate response algorithm without requiring additional intervention by a physician and/or programmer.
In view of the foregoing, it is an object of the invention to provide a rate responsive pacemaker controller that utilizes intracardiac blood temperature as a physiological parameter that provides an accurate indication of physical activity for adjusting pacing rate.
It is another object of the invention to provide such a controller that adjusts the pacing rate in a manner that more accurately approximates the rate response of a normally functioning heart in all metabolic situations by responding to both dynamic temperature increases due to physical exertion and to natural temperature variations related to circadian rhythm, fever, and the like.
It is another object of the invention to provide such a controller that more accurately approximates the rate response of a normally functioning heart by providing gradually adaptive response to varying levels of physical activity.
It is another object of the invention to provide such a controller having an improved rate response algorithm that more accurately approximates the rate response of a normally functioning heart to physical exertion by providing a temporary step-up in pacing rate with the onset of exercise
It is still another object of the invention to provide such a controller providing improved detection of the onset of physical activity in conjunction with the provision of a step rate response.
It is yet another object of the invention to provide such a controller that includes the following additional features: (1) controlling the slew rate of the calculated pacing rate to prevent abrupt rate variations and to provide smoother rate response than prior art systems, (2) monitoring and preventing the occurrence of rate saturation to provide improved accuracy in the rate response, particularly to the cessation of strenuous physical activity, and (3) automatically adjusting the reference temperature utilized by the rate response algorithm to compensate for any error in the initially supplied value of the reference temperature.